1. Technical Field
The invention relates to vibration damping devices which develop damping performance when being applied to a suspension member such as for automobiles or other equipment. More particularly, the invention relates to such a damping device using an electrorheological fluid as the damping medium, which device has a shorter axial height than prior similar damping devices without sacrificing stroke length, and in which the spring rate of the device is minimized during damping by equalizing the effective areas of a pair of elastomeric sleeves which form fluid chambers that contain the electrorheological fluid.
2. Background Information
Vibration damping devices have been used for a considerable period of time to dampen the vibrational forces applied to the suspension system of vehicles to provide a smoother ride by reducing the vibrations caused by road bumps and depressions passing from the tires to the vehicle frame, by the interposing of oil filled shock absorbers or high pressure gas damping devices.
Although these prior art oil and high pressure gas damping devices have proven satisfactory, a more recent development has evolved in which an electrorheological or electroviscous liquid is used within the chambers of the damping device, wherein the liquid is in contact with one or more electrodes usually mounted in a restrictive passage, which depending upon the size of the electrodes and the amount of voltage applied to the liquid, will change the viscosity of the liquid, enabling the damping device to have a greater range of damping characteristics than those achieved by the high pressure gas or oil filled shock absorbers.
U.S. Pat. No. 4,973,031 discloses an anti-vibration apparatus which uses an expandable liquid chamber containing an electrorheological fluid in the chamber.
U.S. Pat. No. 4,858,733 discloses another damping device using electrodes in combination with an electroviscous liquid contained within closed chambers and movable through a restricted passage, where voltage is applied to the electroviscous liquid as it moves through the passage to change its viscosity to achieve various damping effects.
U.S. Pat. No. 5,180,145 discloses another vibration damping device which uses an electrorheological fluid which passes between two closed chambers formed by elastomeric sleeves through a restricted orifice where the fluid comes into contact with an electrode mounted in the restricted passage. The damping device of this patent provides for a shorter axial height than prior similar damping devices without sacrificing stroke length by the piston having a tapered outer surface which can be nested within the hollow interior of a rigid intermediate member through which the electrode extends, as well as permitting the rigid intermediate member to nest within the top annular housing. Thus, it provides a damping device having a shorter axial length without sacrificing stroke length.
However, one problem that exists with these prior art vibration damping devices using the pair of flexible sleeves interconnected through an orifice is that the effective area of the two fluid chambers vary as the fluid moves between the two chambers through the orifice as the damping device moves between jounce and rebound positions. This causes a spring rate effect which affects the vehicle ride and is undesirable for many vibration damping applications. This change in area occurs even if the pair of rolling lobes face each other as shown in U.S. Pat. Nos. 3,025,076, 4,200,270 and 4,518,154, and in pending EPO Application 0 414 508, or extend in the same direction such as shown in U.S. Pat. Nos. 3,627,297 and 4,854,555. This limits the versatility in characteristics desired to be achieved by the damping device for certain applications.
The effective area of a fluid filled elastomeric chamber is defined by the formula: Fluid Volume Displaced divided by the distance that the internal piston moves. In a damper using a pair of fluid pressure chambers separated by an orifice as in the present invention, it is desirable that both elastomeric sleeves replace the same volume for each increment of movement to prevent pressure building within one of the sleeves which results in sleeve stretch, and which causes the sleeve to act as a spring. In prior art damping devices using a pair of fluidly interconnected elastomeric sleeves, the maintaining of this equal effective area within the pair of sleeves was not possible.
Thus, the need exists for an improved damping device which has a pair of fluid chambers formed by flexible elastomeric sleeves connected by an elongated annular orifice, which orifice contains an electrode when used with an electrorheological fluid, which has a shorter axial length than that of similar damping devices without shortening the stroke length; and in which the effective areas of the pair of elastomeric sleeves remain generally equal to provide a relatively low spring rate as the damping device moves between jounce and rebound positions.